DOC (Diesel Oxidation Catalyst), DPF (Diesel Particulate Filter) and NOx occlusion reduction catalyst (LNT: Lean NOx Trap or NSR: NOx Storage Reduction) systems have already been put into practice as exhaust gas aftertreatment devices for diesel engines.
An NOx occlusion reduction catalyst is such that a noble metal catalyst such as Pt, Pd or the like and an occlusion material having an NOx occlusion function such as an alkali metal including Na, K, Cs and the like, an alkali earth metal including Ca, Ba and the like or a rare earth including Y, La, Ce and the like are carried on a catalyst carrier such as an alumina (Al2O3) and exhibits two functions to occlude NOx and to release and purify NOx depending on the oxygen concentration in exhaust gases.
With a purification system utilizing the NOx occlusion reduction catalyst (hereinafter, referred to as an LNT catalyst) which reduces NOx by using the three-way catalyst function, under a condition in which the oxygen concentration in exhaust gases is high (a lean air-fuel ratio) as in a normal operating state, NO in exhaust gases is oxidated to NO2 by a noble metal catalyst such as Pt or Pd, and the occlusion material occludes the oxidated substances as nitrates (Ba(NO3)2) to purify NOx.
When the occlusion of NOx continues, however, since nitrates are saturated to lose the occlusion function as the occlusion material, the operating condition is changed to form a rich state by performing an EGR (Exhaust Gas Recirculation), a post-injection of fuel or an exhaust pipe injection of fuel under a low oxygen concentration condition (a rich air-fuel ratio), so that fuel is reduced on the noble metal catalyst to thereby produce CO, HC, H2 in the exhaust gases so as to reduce released NOx for purification.
In this way, in the purification system using the LNT catalyst, the three-way catalyst function works in which NOx is adsorbed or occluded when the air-fuel ratio is in a lean state (under the high oxygen concentration condition), and the NOx adsorbed or occluded are disorbed or released from Ce or Ba during the rich state, so that HC, CO and NOx in the exhaust gases become harmless by the three-way catalyst function.